1. Field of the Invention
The present invention relates to an endoscope system in which an imaging section is inserted into an object.
Priority is claimed on Japanese Patent Application No. 2010-124663, filed May 31, 2010, the contents of which are incorporated herein by reference.
2. Description of Related Art
In typical circuit systems, an oscillator (which is formed by a crystal oscillator or the like and outputs a highly accurate clock) is provided near to a timing generator which generates timing signals for the circuit, and a clock from this oscillator are used for the base clock which provides the basis of these timing signals. By placing the oscillator close to the timing generator, the base clock can be kept at a high level of accuracy, and it is possible to generate stable timing signals.
In contrast to this, in electronic endoscope systems, in order to make the portion which is inserted inside an organism (i.e., the distal end portion) thinner, and also in order to decrease the power consumption of the distal end portion (i.e., of the distal end portion circuit) and thereby inhibit heat generation, oscillators having oversize packaging and high power consumption are not positioned at the distal end portion. In electronic endoscope systems, it is normal for the oscillator and the distal end portion to be located apart from each other and for a clock to be transmitted from the oscillator to the distal end portion by means of fine-wire coaxial cable, and for this clock to then be used for the base clock of the timing generator of the distal end portion.
However, as the definition of imaging elements has become higher in recent years, the frequency of the timing signals required at the distal end portion has become higher, and it is necessary to raise the frequency of the source oscillation clock for the timing generator of the distal end portion. In conjunction with this, the frequency of the transmitted oscillator clock is also raised. As a consequence, if the oscillator clock is transmitted using a fine-wire coaxial cable, the signal quality of the clock deteriorates before it reaches the distal end portion, and there is a possibility that operations of the system will be destabilized. There may also be instances of increased noise radiation due to higher harmonic waves.
In Japanese Patent Application, First Publication No. 2009-45113 a method has been proposed in which, using a twisted pair cable, a high-speed oscillator clock is transmitted by means of a differential digital signal. According to this method, because the clock signal has a small amplitude, relatively high-speed signal can be transmitted, and it is also possible to reduce noise radiation.
Moreover, if a system such as that described in, for example, Japanese Patent Application, First Publication No. 2008-154934 in which video signals undergo A/D conversion in the distal end portion, and are transmitted as digital signals is considered, then the timing signals which are necessary within the circuit in the distal end portion become considerably faster and an extremely high level of accuracy is sought in the base clock on the basis of which such timing signals are generated. As a consequence of this, there is a possibility that the oscillator clock which is transmitted by means of a twisted pair cable will not have sufficient accuracy. Moreover, if the clock having the required speed cannot be obtained using the transmitted oscillator clock, then a multiplier circuit may be provided in the timing generator in the distal end portion and a high-speed clock is created from the transmitted base clock, however, depending on the type of multiplier circuit used, there may be instances in which the clock signal quality becomes even more deteriorated.
A multiplier circuit is generally formed by a phase synchronization circuit having a feedback mechanism which uses a phase-locked loop (PLL) or a delay-locked loop (DLL) (see, for example, FIG. 7 in Japanese Patent Application, First Publication No. 2004-221962, and FIG. 12 in Japanese Patent Application, First Publication No. H10-215153). Such phase synchronization circuits are divided into a type in which all of the systems that make up the feedback mechanism are formed within the same integrated circuit (i.e., an all-digital type), and a type in which a portion of the systems of the feedback mechanism is formed as a separate device thereby enabling highly accurate signals to be obtained (i.e., an external attachment type). In such phase synchronization circuits, if noise and the like become superimposed on the wiring of the feedback systems (i.e., on the wiring forming the feedback loop) so that the signal periodicity is disturbed, then there is a possibility that the synchronization will be out and that the circuit operations will become destabilized, and in some cases considerable time may be lost until stable operations are restored.
All-digital type circuits have all of the feedback systems formed within the same integrated circuit so that these circuits are consequently small in size. Moreover, because the wiring of the feedback systems is not located outside the integrated circuit in an all-digital type, it is possible to reduce the effects of noise. Typically, all-digital types have poor oscillation characteristics and it is possible that there will be a large amount of jitter (i.e., temporal displacement of the signal). For example, in the PLL of an all-digital type, typically, a ring oscillator (RO) is used as a voltage control oscillator (VCO), however, RO generally have poor jitter characteristics, and there are instances in which, unless the base clock is extremely accurate, it is difficult to handle high-frequency signals.
In external attachment types, for example, in a PLL, by employing a highly accurate, externally attached voltage control crystal oscillator (VCXO) as the VCO, it is possible to greatly improve the oscillation characteristics. However, as a result of using a separate device, the size easily increases and the wiring that connects the feedback systems together also extends outside the integrated circuit so that, in some instances, the effects of noise also increase. Size limitations make it impossible to provide a strong magnetic shield, and the fact that this external attachment type of multiplier circuit is located in the distal end portion where it is difficult to avoid the effects of surrounding noise sources means there is a possibility that the system might become destabilized. Note that in medical environments in which an endoscope is used, devices that generate excessive noise such as electrosurgical units and the like are operating at the same time as the endoscope is being used. Consequently, noise is scattered around at a level not present in normal environments, and the effects of such noise are extremely large.